The metro card for getting around: Bartik (I think it's called off the top of my head!) like an Oyster card, costs €3, up to 10 people can travel on it, it includes metro, bus, funicular, all sorts. We went to Burmeo an hour+10 away for 84cents, then a bus to Gaztelugatxe for 34c each! Crazy cheap!
Gaztelugatxe is free, but must book a time and arrive ten minutes before. One bus an hour, from Burmeo, so better by car. Easy to find.
Guggenheim is €18 adults, under 15 (I think - you need to check) is free, but yes book ahead for August, I think you can beat the Q of people who are paying on the day.
After Gaztelugatxe we went back on the train to Gernika. The museum (€6 adults, free for DD age 12), is very humbling, very emotional visit. We went after for a mooch about afterwards and had tapas - the main placa was heaving with all ages, people enjoying themselves - it felt like a real 'fuck you, Franco' to the 1937 bombings!! It's a small town absolutely full of life, we loved it there.
We went down to the beach at Algorta (just round the headland from Getxo) and it was ok, dark river water, there were no lifeguards or flag notifying safety for swimming. Getxo beach might be an entirely different kettle of fish. Bear in mind it's the Atlantic, so might be chilly even in August.
We have all felt very safe walking around at all hours and catching public transport - nothing like Barcelona (which we know well and it teaming with scuzzy thieves). We're keeping an eye on bags as always, but it feels very safe.
Everyone has been delightful - in shops, buses, randoms explaining parts of the city, just lovely friendly people.
Enjoy in August!